Leaky mode systems may be used for holographic video, e.g., flat-screen, scanned aperture, and near-eye holographic video systems. In a leaky mode system, which generally comprises multiple leaky mode devices, surface acoustic waves (“SAW”) for a leaky mode device are generated by a transducer that encodes electrical information as a pattern of surface acoustic waves. This surface acoustic wave pattern acts both to mode couple light so that it is no longer guided and also to encode the light with holographic information. The leaky mode light propagates to form part or all of a holographic image.
One of the shortcomings in leaky mode light systems is that light exits the edge, rather than the bottom, of the device. This means that the device output aperture depends on the device thickness. Larger apertures mean thicker, more expensive, harder-to-process devices. Because in existing leaky light mode devices light exits the edge, but not the bottom, such devices can be combined only in one-dimensional arrays (which might themselves be one dimensional arrays). Otherwise, the light exit would be blocked. FIG. 6 shows an example of a one-dimensional array 600 of leaky mode devices 605, with respective transducers 610, respective SAWs 620, and respective light 630 exiting from side of leaky mode device 605n. 
Additionally, fabricating gratings in leaky mode devices can be difficult and expensive when gratings have high spatial frequency. A grating in a leaky mode device is a periodic structure, usually etched into the bottom surface of the leaky mode substrate. The period of these gratings is typically around 300 nm. This grating structure takes leaky mode light traveling at approximately 10 degrees internally and bends it to 90 degrees as it exits the bottom surface of the device substrate. Standard photolithographic processes can be used to create patterns down to 1 μm features size. However, output gratings in lithium niobate meant to outcouple leaky mode light typically have grating periods around 300 nm. At this feature size, interference lithography, contact lithography, or more commonly, ebeam lithography must be used. But these fabrication techniques are difficult, expensive, and suffer from other shortcomings. Interference lithography has limited control of the grating pattern (only uniform gratings). Contact lithography requires special, thin, fragile flex masks that degrade with use. Ebeam lithography writes the pattern serially and is considered a low-throughput, high cost technique.
Another problem is that a bottom exit grating will reduce the field of view of a leaky mode near-eye display as compared with an edge-exit leaky mode display. Leaky mode modulators change the angle of light during mode conversion by adding or subtracting the spatial frequency of a surface acoustic wave to the spatial frequency of the light in the guide input mode. A grating output coupler adds or subtracts spatial frequencies in opposition to the grating formed by surface acoustic waves, essentially undoing some of the angular deflection.
Additionally, even when bottom exit gratings are used to direct and guide light to exit out the bottom of a leaky mode device, when a user moves it ay appear that that a virtual point is shifting with the user, rather than appearing to the user that the virtual point remains at the same point in space regardless of how a user has shifted,
What is needed is improvements to leaky mode devices to facilitate non-grating light exit from the bottom of a leaky mode device and/or to mitigate and/or overcome edge exit for leaky mode light devices, and further to mitigate obstacles and other issues associated with bottom exit in leaky mode devices.